Self-baking electrodes for electric arc furnaces

ABSTRACT

A self-baking electrode of the Soderberg type for electric arc furnaces comprises an outer metal sleeve containing the electrode mixture and internal reinforcing members for the electrode mixture, said reinforcing or structural members being electrically insulated from the outer metallic sleeve. The internal wall of the sleeve preferably is provided with radial ribs extending the whole length of the sleeve and the reinforcing members are attached to the ribs by means of insulating strips.

United States Patent Inventor Bruno Orlando Milan, Italy Appl. No859,428 Filed Sept. I9. [969 Patented July 27. [97] Assignee Kinglor.Finanz-und Beratungsanstal Eschen, Liechtenstein Priority Nov. 28, 1968Switzerland 17,784/68 SELF-BAKING ELECTRODES FOR ELECTRIC ARC FURNACES 4Claims, 4 Drawing Figs.

US. Cl 13/18 lnt.Cl "05b 7/06 FieldofSearch.. 13/18, 18 SOD [56]References Cited UNITED STATES PATENTS 2,666,087 l/l954 Johansson et al.l3/l8 (SOD) 3,365,533 l/l968 Alexander l3/18 (SOD) PrimaryExaminer-Bernard A. Gilheany Assistant Examiner-R. N. Envall, Jr.Attorney-Anderson, Luedeka, Fitch, Even and Tabin ABSTRACT: Aself-baking electrode of the Siiderberg type for electric arc furnacescomprises an outer metal sleeve containing the electrode mixture andinternal reinforcing members for the electrode mixture, said reinforcingor structural members being electrically insulated from the outermetallic sleeve. The internal wall of the sleeve preferably is providedwith radial ribs extending the whole length of the sleeve and thereinforcing members are attached to the ribs by means of insulatingstrips.

PATENTED JULZ'! IS?! Fig.1 PRIOR ART Fig. 2 PRIOR ART furnaces.

SELF-BAKING ELECTRODES Fort ELECTRIC ARC v FURNACES The known electrodesof the type referred to, particularly those for electric submerged arcfurnaces for the production of ferroalloys, calcium carbide,- silicon,electrofused ox-ides, I

typically consist of a metallic sleeve, such as a cylindrical sleeve,which is filled with a crude or green carbonaceous electrode mass. Theseelectrodes are vertically positioned above the crucible of the furnaceproper and their lower portions are exposed to the high temperaturewithin the crucible. Because of this high temperature, thegreen'electrode mass is subjected to a baking process which transformssaid green mass into a solid carbonaceous body which is highlyrefractory and has a relatively high electrical conductivity. The bakedportion of the electrode mass acquires the shape of the shell, whereasthe latter is continuously melting away owing to the high temperaturesin the baking region of the electrode.

In a self-baking electrode there are consequently, as seen from theirbottom end upwardly, a solid portion with a relatively high electricalconductivity and having temperatures ranging between over 2,000 C. andabout 800 C.; this solid portion being followed at its upper end by abaking zone in which the baking process is taking place, i.e. in whichthe electrode mass is semifluid and, therefore, a poor conductor, thetemperatures in this baking zone ranging from approximately 800 C. to300 C. ln the zone above said baking zone there is a zone in which theelectrodemass is fluid, said zone having temperatures ranging from about300 C. to 100 C., and in which the electrode mass does not conductelectricity.

Finally, in the uppermost portion of the electrode there is a zone inwhich the electrode mass is particulate and has tem peratures belowabout 100 C. and in which, of course, the electrode mass is not aconductor of electric energy.

The supply of electricity to the electrode is effected by metallicconductors such as bus bars which contact the electrode shell betweenthe solid zone or portion and the baking zone of the electrode Theseconductors generally consist of buses of copper or bronze having a largecontact surface and According to a known practice, the supportingstructural 'members'inside the sleeve consist in metallic strips orvanes 2 which are welded to the inside wall of'the sleevel in radialposition as shown in FIGS. 1 and 2 of the drawings. These strips orwings 2 comprise openings 3 or beads 4 for anchoring the baked electrodemass. Therefore,'the weight of the baked solid portion and of theelectrode mass lying on'it is supported by these strips or wings 2,which transfer the weightto the metallic sleeve, which in turn iscarried by suitable carrying or supporting means. I

There are several patents describing particular designs of supportingstructural members for self-baking electrodes.

are generally thoroughly cooled in order to withstand the hightemperatures in this zone. Because of this thorough cooling of the busesand of their contact surfaces respectively, the electrode temperature inthe contact zone is also limited, and consequently the baking zone ofthe electrode is prevented from passing the contact zone.

However, the metallic sleeve and the electrode mass being the onlycomponents of the electrode assembly, same has, therefore, to bereinforced inside the sleeve by supporting structural members. In fact,the weight of the solid baked portion of the electrode and of theelectrode mass lying on it may not be totally supported by the busesand, if no other supporting means are provided, the solid portion andthe electrode mass lying on it often fall by their own weight into thecrucible of the furnace The present invention will be describedhereinafter in connection with the accompanying drawing in which:

FIG. 1 is a partial sectional view taken axially along the length of aprior an electrode with the electrode mass having been omitted for thepurpose of clarity;

FIG. 2 is a cross-sectional view taken along line ll-ll of FIG.

FIG. 3 shows a partial sectional view taken axially along the length ofan electrode, the electrode mass having been omitted for clarity,embodying the novel features of the invcn tion; and

FIG. 4 is a cross-sectional view taken along the line lV-lV of FIG. 3.

Some of these known designs are particularly adapted for electrodes forthe production of aluminum: the electrode weight is carried by metalbars embedded in the electrode, said bars also serving to supplyelectric power. This known design is not suitable for self-bakingelectrodes for electric arc furnaces because of the different operatingconditions (such as temperatures, stresses, density of electric current,power density) and also because electric arc furnaces usually operate onAC power and the length of the embedded bars would cause an undesiredand harmful increase of electrode impedance.

' Other known designs show particular shapes of the shell or of thestructural members associated therewith, but these designs have otherobjects than those of the present invention.

During its operation in the furnace, the electrode is consumed, i.e.more specifically is consumed at its lowermost portion. it is thereforenecessary to lower periodically the electrode relative to the buses tomaintain the electrode length below the zone of contact of said buses.As stated above, the buses are connected to the electrodes between thebaked zone (i.e..conducting zone) and baking zone (i.e. nonconductingzone) of the electrode -mass. When the electrode is appreciably loweredrelative to the buses to compensate for wear by consumption, it mayoccur that the lowered electrode mass is not yet a conductor or is apoor conductor. In such cases, the major portion of the power suppliedto the electrode by the buses has to pass the metallic sleeve, fromwhich it passes to the structural members (which constitute thesupporting elements), and finally passes from the structural members tothe baked zone of the electrode. .lf now the structural members do nothave a sufficiently large cross-sectional area to support thehigh-current densities, they will become red hot because of the Jouleeffect, thus losing their mechanical strength and becoming unable tosupport any longer the electrode weight, and consequently causingbreakage or failure of the electrode.

Failure of the type described is a frequent occurrence in the practicaluse of self-baking electrodes, because, in fact, the cross-sectionalarea of the structural members is not sufficient to support theelectrode current. Typically, an electrode having an outside diameter of1,000 mm. has a normal current loading of about 50,000 a., and thestructural members, being usually made of steel by reason of mechanicalstress, would have to be designed to have a total cross-sectional areaof about 2,500 em. if the were to safely support the nominal currentloading, thus occupying about one-third of the whole electrode crosssection.

ln order to minimize the risk of electrode breakages of the typereferred to, it is necessary to adjust the electrode length below thecontact surface in very small steps, typically some centimeters at atime, in order to ensure that the lower part of the bus bars is alwayscontacting part of the baked zone of the electrode.

of course, such frequent and small adjustments are very detrimental toefficient operation of the furnace while they do not eliminate totallythe risk of electrode breakage.

Moreover, it occurs frequently that the electrode is consumed rapidlywhich results in a too short overall electrode length for efficientfurnace operation, while a length adjustment is not possible by thedanger of electrode breakage as described above. in such cases, thelowermost tip of. the eleclt is, therefore, an object of the presentinvention to provide.

an electrode of the type referred to in which the drawbacks describedare substantially eliminated.

With this and other objects in mind, the invention provides in aself-baking electrode for electric arc furnaces, particularlyfor'submerged arc furnaces, said electrode having a metallic sleeve andinternal structural member means affixed to said metallic sleeve, theimprovement of an electric insulation means insulating the internalstructural member means form the metallic sleeve.

In this way, the internal structural members support the weight of thebaked portion of the electrode and of the electrode mass overlying it.These structural members being affixed to the sleeve transfer by theelectric insulation means the electrode weight to the metallic shell asin the known electrodes, but the insulation means between the structuralmembers and the sleeve prevents in any case the structural supportingmember from being electrically overloaded and, consequently, from beingoverheated to such an extent so as to lose the mechanicalcharacteristics.

In this way the danger of electrode breakage is completely eliminated.

When an electrode is adjusted in length for compensating its wear byconsumption, it might occur that the power supplying bus bars areconnected to a zone of the electrode which is still a poor electricalconductor: in this case, the internal structural members are notaffected because they are protected by their electrical insulation. inthe zone of the electrode having a poor electrical conductivity a highervoltage drop occurs than in other zones of the electrode, and, becauseof this voltage drop and of the current, a considerable quantity of heatis developed causing, in short time, this zone of the electrodecontacted by the bus bars to be completely baked. This possibility ofrapid baking allows for adjusting more frequently and by longer stepsthe electrode length and prevents, therefore, the risk of furnaceoperation with excessively short electrodes, thus allowing a furnaceoperation athigher power rating.

lt'has been found, by way of example, that applying the invention toelectrodes having a diameter of about 1,000 mm. adjustments in lengthmay be safely carried out twice as frequently and by twice the amountwithout any risk of electrode breakage.

Moreover, it has been found that with the inventive concept it ispossible to considerably decrease (by about half with respect to theconventional electrodes) the total cross-sectional area of the internalstructural members, such as of the strips constituting such members:this was to be expected because of the fact that in no case thesestructural members are subjected to the passage of electric current andare, therefore, in no case overheated to such an extent that theirphysical strength is diminished.

Accordingly, it has been found, for example, that in an elec- 1 trodehaving a diameter of 1,000 mm. it is possible to reduce the number ofradial strips forming the internal structural members from eight stripsto four without incurring any risk of electrode breakage.

An embodiment illustrative for the invention is shown in F108. 3 and 4.

ln FIGS. 3 and 4 the outer metallic sleeve is indicated by the numeral5. In the interior of sleeve 5 four radial ribs 6 (only three shown) areattached such as by welding the extend along the whole length of thesleeve 5. To'each of these ribs 6 there is affixed a strip 7 of aninsulating material such as wood, cardboard, fiber or micanitc. Thesestrips may be affixed by any suitable means, such as by bonding with anadhesive.

To each of the strips 7 there is affixed another strip 8, the strips 8forming the internal structural members.

As in the conventional electrodes, the strips 8 have formed thereinapertures 9 (FIG. 4) or beads 10 which serve as anchoring means of thestrips 8 in the carbonaceous electrode mass (not shown).

It is understood that instead of four ribs 6 and four strips 8 there maybe provided a higher or lower number according to the internal sizeofthe sleeve and/or according to the physical strength of the insulatingstrips 7 and of the adhesive joints.

' Moreover, each of the strips 8 may be affixed to the ribs by means ofa pair of insulating strips 7 bonded or glued to either side of thestrips 8 and of the ribs 6. The strips 8 may be fastened to the sleeve 5by means of the profile, such as a T- profile of an electricallyinsulating material.

Finally, the internal structural members need not necessarily be in theform of s strips 8 as shown. The internal structural members may beformed by a wire mesh either shaped as lengthwise-running ribbons or asconcentric inner tube connected with and insulated by any suitable meansto the outer metallic sleeve.

What I claim is:

l. A'self-baking electrode for electric furnaces comprising a metallicouter sleeve containing an electrode mixture, structural members withinsaid sleeve, and insulating means secured to the inner side of saidsleeve and to said structural members for electrically insulating saidmembers from said sleeve.

2. The electrode of claim 1 in which the internal structural members areaffixed along their whole length to the outer metallic sleeve.

3. The electrode of claim 2 in which the internal structural members areformed of metallic strips, said strips being disposed inside saidsleeve, said insulating means comprises strips of an insulating materialand in which said metallic strips are affixed to the outer sleeve bymeans of said strips of an electrically insulating material.

4. The electrode of claim 3 in which the outer metallic sleeve is acylinder or prismatic tube carrying said metallic strips, said metallicstrips being disposed radially and having affixed thereto said strips ofelectrically insulating material.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,595,977 Dated July 27, 1971 Inventor(s) Bruno Orlando It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 2, line 55 "50,000 a.," should be 50,000 Amps, Column 2, line 59"if the were" should be if they were. Column 3, line 12 "form" should befrom. Column 4, line 9 "welding the extend should be welding and extend.Column 4, line 27 "of the profile" should be of a profile-. Column 4,line 30 "of s strips" should be of strips. Column 4, line 52 Claim 4"cylinder" should be cylindrical.

Signed and sealed this 7th day of March 1972.

(SEAL) fittest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Afi5est1ng Officer Commissionerof Patents DRM P O-1050 [IO-69 USCOMM-DC 60376-P69 fr U '5 (-IIVERNMENYPIHNTiNG OFFICE 1969 O- 3b6 33A

1. A self-baking electrode for electric furnaces comprising a metallic outer sleeve containing an electrode mixture, structural members within said sleeve, and insulating means secured to the inner side of said sleeve and to said structural members for electrically insulating said members from said sleeve.
 2. The electrode of claim 1 in which the internal structural members are affixed along their whole length to the outer metallic sleeve.
 3. The electrode of claim 2 in which the internal structural members are formed of metallic strips, said strips being disposed inside said sleeve, said insulating means comprises strips of an insulating material and in which said metallic strips are affixed to the outer sleeve by means of said strips of an electrically insulating material.
 4. The electrode of claim 3 in which the outer metallic sleeve is a cylinder or prismatic tube carrying said metallic strips, said metallic strips being disposed radially and having affixed thereto said strips of electrically insulating material. 